Protein Trafficking And Degradation

Question 1

Ribosomes are directed to bind to ER when involved in synthesizing products destined for ____

Answer 1

. Insertion into cell membranes
. Function w/in lysosomes
. Secretion outside cell

Question 2

Free ribosomes synthesize proteins for what locations?

Answer 2

. Nucleus
. Mitochondria
. Peroxisomes

Question 3

Signal sequences

Answer 3

. Structural features found w/in sequences of protein being produced
. Recognized by organelles
. Direct protein o appropriate location for further modification to make protein functional

Question 4

Proteins synthesized on bound ribosomes move ___

Answer 4

Outside cell

Question 5

Proteins synthesized on free ribosomes are located ____

Answer 5

W/in the same cytosol

Question 6

N-terminal hydrophobic signal sequence (NH2-terminal signal/leader sequence)

Answer 6

. Presence on newly synthesized protein causes ribosome synthesizing it to bind to ER
. N-terminal: amino terminal region of protein
. Hydrophobic: contains AA that don’t interact w/ water
. Recognized by signal recognition particles (SRPs)

Question 7

Signal recognition particles

Answer 7

. Complex made of proteins and RNA
. facilitate attachment of ribosome to ER
. SRP bound to leader sequence bind to a SRP receptor on membrane of ER
. Once ribosome is docked to ER membrane, the nascent polypeptide enters into ER lumen

Question 8

N-glycosylation

Answer 8

Once in ER, leader sequence is cleaved by proteases
. Remainder of AA sequences enters space of lumen
. most proteins w/ Asn-X-Ser or Asn-X-Thr have 14 sugar, branched oligosaccharide added to it from membrane lipid dolichol
. Co-translational

Question 9

Transitional element

Answer 9

. Area of smooth ER where proteins go through a series of membrane spaces w/in ER
. Membrane of transitional element surrounds and encloses nascent polypeptide until it buds off into a transport vesicle
. Vesicle fuses w/ Golgi

Question 10

Vesicles from smooth ER fuse with what part of Golgi?

Answer 10

Cis-golgi

Question 11

Processes w/in Golgi

Answer 11

. Glycosylation: addition of carb
. Sulfation: addition of sulfur
. Phosphorylation: addition of phosphate
. Proteolysis: cleavage of peptide bonds

Question 12

Mannose-6-phosphate tag

Answer 12

. Occurs on proteins that will function in lysosome

. Phosphorylated on mannoses added to the protein

Question 13

Trans golgi network

Answer 13

. Last sorting and packaging region of golgi

. Polypeptides sent to lysosomes or out of cell

Question 14

Lysosome proteins and characteristics

Answer 14

. Acidic internal pH
. Degrade unwanted macromolecules
. Contains hydrolases that degrade unwanted macromolecules at acidic pH
. Lysosomal membrane keeps hydrolases compartmentalized and sequestered from cytoplasmic macromolecules

Question 15

How trafficking of acid hydrolases works

Answer 15

. Addition of mannose-6-phosphate to precursors
. Segregation of those precursors in trans golgi by binding to mannose-6P receptors clustered in clathrin-coated pits
. Formation and using of clathrin-coated vesicles from TGN
. Fusion of transport vesicle w/ endolysosomes and loss of clathrate coat
. Dissociation of hydrolases from mannose-6P receptors because of acid pH of endolysosome
. Removal of phosphate via phosphatase
. Recycle receptors

Question 16

I cell disease

Answer 16

. Problem of targeting all acid hydrolases to lysosomes

Question 17

Types of lysosomal storage diseases

Answer 17

. Hunter and hurler
. Tay Sach’s
. Gaucher

Question 18

Pathway of proteins destined for secretion outside o the cell from the TGN

Answer 18

. Bud off from TGN encapsulated in transport vesicle

. Fuses w/ plasma membrane (constitutive) or stored in cytoplasm until release (regulated)

Question 19

Default pathway for proteins synthesized on free ribosomes

Answer 19

. Remain in cytosol

. Structural features can be incorporated to direct these proteins to specific organelles

Question 20

Cytosolic proteins

Answer 20

. Intracellular proteins that function outside boundaries of organelles

Question 21

Nuclear proteins

Answer 21

. Contain nuclear localization signal (NLS) that allows them through nuclear pore
. Binds strongly to importin (receptor on nuclear envelope)
. Involved in DNA replication or RNA Synthesis

Question 22

Mitochondrial proteins

Answer 22

. Contain N-terminal mitochondrial import sequence
. Nuclear proteins synthesized on free ribosomes associate w/ chaperone proteins to keep them unfolded prior to entry in mitochondria
. TOM and TIM have proteins cross through outer and inner mitochondrial membranes and into the matrix

Question 23

Peroxisomal proteins

Answer 23

. Hydrolytic enzymes
. Contain C-terminal tripeptide that functions as peroxisomal targeting signal
. ER-derived
. Breakdown fatty acids and purines (AMP/GMP)
. Detox hydrogen peroxide
. Cholesterol and bile synthesis in hepatocytes
. Synthesis of myelin

Question 24

What are some inherited diseases caused by peroxisomal dysfunction?

Answer 24

. X-linked adrenoleukodystrophy

. Zellweger syndrome

Question 25

Insulin biosynthesis

Answer 25

. Translation of mRNA gives preproinsulin w/ signal peptide that directs the protein into ER lumen
. Proteolytically cleaved to yield proinsulin inside the ER
. Proinsulin transported to golgi and proteolytically cleaved into insulin and C-peptide
. Insulin stored in cytosol in granules or vesicles
. Insulin released via regulated exocytosis

Question 26

Protein has a finite life span is eventually degraded in order to _____

Answer 26

. Supply AA for new protein synthesis
. Remove transcription factors, enzymes or any other protein whose activity isn’t needed
. Prevent accumulation of abnormal proteins

Question 27

Pathways to degrade proteins

Answer 27

. Lysosomal

. Proteasome

Question 28

Lysosomal degradation pathway

Answer 28

. Degrades membrane and extracellular proteins via endocytosis
. Degrades proteins w/ extended half-lives
. Occurs via autophagy (autophagosome vesicles engulf small amounts of material from cytoplasm or organelles using fragments of ER membrane)
. Vesicles fuse w/ lysosomes triggering release of hydrolytic enzymes and degradation proteins

Question 29

Proteasome pathway

Answer 29

. Degrades intracellular proteins w/ short half-lives
. ATP dependent
. Get ubiquinated, once 4 ubiquitin in chain make it a target for proteasome
. Proteins recognized and degraded by 60-protein complex (26S proteasome)
. Proteasome ha central core (20S particle) of 7 beta-subunits and 2 outer rings (19S particle) w/ 7-alpha subunits
. 19S particles recognize proteins, remove ubiquitin, unfold, and translocate protein to core to be hydrolyzed into peptides
. Peptides exit proteasome and are degraded in cytosol

Question 30

Process of ubiquitination of proteins

Answer 30

. Ubiquitin: 76 AA protein
. Ubiquitin-activating enzyme (E1) bind ubiquitin and transfers it to ubiquitin-conjugating enzyme (E2)
. Ubiquitin ligase enzyme (E3) promotes transfer of ubiquitin from E2 to Lys residues of protein recognized by E3 to be degraded

Question 31

Proteins w/ long half life

Answer 31

. N-terminal Met, Ser, Ala, Thr, Val, or Gly: half life of 20 hours

Question 32

Proteins w/ half life less than 3 minutes

Answer 32

. Phe
. Leu
. Asp 
. Lys
. Arg

Question 33

Proteins that are rapidly degraded

Answer 33

. Proteins rich in Pro, Glu, Ser, and Thr (PEST)

Question 34

Bortezomib

Answer 34

. Blocks proteolytic action of proteasome
. Used to treat patients w/ multiple myeloma
. Inhibits degradation of several proteins involved in cell cycle control and apoptosis